Silver powder made of silver particles, each to which fine silver particles adhere and process of producing the same

ABSTRACT

This invention is a silver powder having a low-temperature sintering performance and dispersibility, which allows the powder particles to be agglomerated to a small degree and be nearly in the monodisperse state. Employed is silver powder of fine silver particles each to which fine silver particles adhere, wherein fine silver particles of nano-order particle size are adhered to the surface of each silver powder particle. The powder particles of the silver powder of fine silver particles each to which fine silver particles adhere have excellent dispersibility. In the production of the silver powder of fine silver particles each to which fine silver particles adhere, a process of including the steps of: adding a silver nitrate and a neutralizing agent into a slurry of silver powder in a dispersing medium; dissolving the mixture while stirring to allow fine silver oxide particles to be precipitated on the surface of each silver powder particle; washing the resultant silver powder; and exposing the fine silver oxide particles to UV rays to reduce the same to fine silver particles.

TECHNICAL FIELD

The present invention relates to a silver powder made of silverparticles, each to which fine silver particles adhere and a process ofproducing the same.

BACKGROUND ART

Silver ink (silver paste) has been conventionally used not only forforming circuit boards by co-firing it with a ceramic substrate at arelatively high temperature but also for forming a wiring circuit on aprinted wiring board, via-hole-fillers through such a board andadhesives for mounting electrical parts thereon by mixing and curing thesilver ink (silver paste) with various types of resin ingredients, asdescribed in the Patent Document 1. In the latter application, ingeneral, an electrical conductivity has been obtained simply by mutuallyhaving the particles of silver powder touched, as a conductive fillerwithout mutually sintering the particles.

However, there have been a demand in recent years for lowering anelectrical resistance for conductor portions formed using the silverpowder and improving connecting reliability in order to realize loweringof the electrical resistance, and consequently, more demand has occurredfor silver ink or silver paste to be formed by sintering a filler itselfof the silver powder while resin ingredients are being cured, resultingin that the conductivity is exhibited. It goes without saying that formeeting these demands, size-fining of each of the particles of thesilver powder as a conductive filler is necessary in order to lower thesintering temperature.

Conventionally, a wet reduction process where an aqueous solution of asilver/ammine complex using a silver nitrate solution and an aqueousammonia is produced; and an organic reducing agent is added to theaqueous solution of the complex, as described in the Patent Document 2,has been employed for production of a silver powder, and the obtainedsilver powder has been made into a silver paste. Additionally, in orderto ensure more excellent low-temperature sintering performance thanconventional, the silver ink containing silver nanoparticles, asdisclosed in the Patent Document 3, has been proposed.

[Patent Document 1]

-   -   Japanese Patent Laid-Open No. 2001-107101

[Patent Document 2]

-   -   Japanese Patent Laid-Open No. 2002-334618

[Patent Document 3]

-   -   Japanese Patent Laid-Open No. 2002-324966

DISCLOSURE OF THE INVENTION

However, in a metal powder such as a silver powder, it is generally saidthat both the size-refinement of powder particles and the dispersibilitymeant by that powder particles nearly in a mono-disperse state are hardto become satisfied. For example, in the case of the silver inkcontaining silver nanoparticles, as disclosed in the above-describedPatent Document 1, in order to stabilize a state of the dispersion ofthe nanoparticles, it is common to add a large amount of dispersant as aprotective colloid thereto. In such a case, generally the decompositiontemperature of the dispersant is higher than the sintering temperatureof the silver nanoparticles, resulting in that the low-temperaturesintering performance of the silver nanoparticles themselves cannot beused to the full.

Further, in case of the silver ink containing the silver nanoparticles,because the content of the filler is less than conventional, a thickfilm is difficult to form while a thin film is easy to form. This makesit difficult to use such silver ink for application to form wiringcircuits having a large cross-sectional area sufficient to be usable forpower-supply circuit such carries a relatively large amount of currentor for application to form a low-resistance circuit. Further, in the useof the silver ink for adhesives for mounting components thereon, notonly its conductivity, but also its adhesive strength is requiredseverely. Therefore it is indispensable to add a certain amount or moreof resin thereto which exhibits high-adhesive strength when the resin iscured. Thus, there remain many problems that cannot be overcome byconventional silver-nanoparticles-containing silver ink.

In the meantime, it goes without saying that silver powder used forcommonly-used silver pastes has had a limitation in terms of itslow-temperature sintering performance judging from its particle size.Because in the particle-size of the silver powder obtained by theconventional production processes, the actual condition is that theaverage particle size between their primary particles D_(IA) is usuallymore than 0.6 μm, the average particle size D₅₀ measured by a laserdiffraction scattering particle size distribution measurement method ismore than 1.0 μm, and the degree of agglomeration represented byD₅₀/D_(IA) is more than 1.7 (Note that “D_(IA)” is a particle-size whichcan be calculated by a particle image from an SEM image analysis.).Thus, the conventional silver powder has been unsuitable when formingrecent circuits where fine-pitched wiring patterns are drawn.

Further, in the circuit formation in a case where a silver paste usingthe conventional silver powder is employed, non-firing type applicationor low-temperature sintering type one is often used where heatingtemperature is 300° C. or less. In such circumstance, the use of silverpowder having low crystallinity has been considered to be preferable, inorder to obtain high sintering performance at a low temperature.However, in order to obtain silver powder of low crystallinity, it isinevitable to employ a reaction system where reduction is rapidlyadvanced in view of its manufacturing condition. As a result, a silverpowder has been merely obtained such that its crystallinity is low, andadditionally its agglomeration significantly occurs easily. Thus, in amarket, there have been demands for silver powder which has alow-temperature sintering performance, whose particles each is finerthan that of particles of any conventional silver powder, and which hasan excellent dispersibility such that little agglomerate occursregarding powder particles.

Also on the other hand, a silver powder has been required to have fewimpurities. In the production of the silver powder, the above-describedwet reduction process has been employed, and the reducing agent, etc.used in the process remains on the surface of the powder particles.Thus, as long as the conventional production process is employed, theproblem regarding the impurities is inevitable. And there has occurredanother problem that in accompanying with the increase of the amount ofimpurities on the surface of the silver powder, an electrical resistanceof the conductor formed using the silver powder also becomes higher.

Thus, there have been demands in the market for a silver powder whichhas an excellent low-temperature sintering performance which has notexisted until now, whose particles each is very fine and have highdispersibility, and which has little impurities in order to realize alow electrical resistance.

In the light of the above-described problems, the present inventors havemade eager research efforts as to any novel process for fining of eachof particles of the conventional silver powder particles themselves.However, it is natural that the currently-used technology has someinherent limitations at a level. So then the inventors suppose that whensintering the silver powder particles, an only outer portion of surfaceof each of silver powder particles may be sufficiently sintered andconnected. Further, if so, they suppose that even the above-describedconventional silver powder could have a low-temperature sinterablecharacteristic. Hereinafter, the present invention will be described interms of two separate points: “silver powder made of silver particles,each to which fine silver particles are adhered” and “process ofproducing the silver powder”.

<Silver Powder Made of Silver Particles, Each to which Fine SilverParticles Adhere>

The “silver powder made of silver particles whose center part isregarded as a core material, in which silver particles each being finerthan the silver particle of the center part adheres to the corematerial”(i.e., silver powder made of silver particles, each to whichfine silver particles adhere) relating to the present invention is inother words expressed by “silver powder made of silver particles eachbeing made adhered to the surface of each of the silver powderparticles”. Specifically, the surface of each powder particle of thesilver powder 2 as a core material is further coated with much finersilver particles 3, just like the image shown in FIG. 1. Thus, the finesilver particles 3 existing on the surface of each particle of thesilver powder 2, each allows the fine silver particles 2 to exhibit alow-temperature sintering performance, independent of the shape and sizeof the powder particles of the core material, thereby making easier thesintering of the adjacent powder particles of the silver powder made ofsilver particles 1 made to adhere thereto.

The term “fine silver particles” herein used means silver nanoparticlesof nanometer-order particle size which exist only on the surface of eachparticle of the silver powder 2. As above-mentioned, when using thesilver nanoparticles themselves for a silver ink, generally a largeamount of dispersant whose decomposition temperature is higher than thesintering temperature of silver nano-particles, is added to stabilizethe dispersibility of the nanoparticles. As a result, the characteristicof being sintered at low temperatures which silver nanoparticlesthemselves have cannot be used to the full. However, having much finersilver particles 3 additionally adhered onto the surface of each powderparticle of the silver powder 2 enables the characteristic of beingsintered at low temperatures which silver nanoparticles have to besufficiently used, independent of the size and shape of the powderparticles of the silver powder as the core material. Accordingly, evenif the shape of the silver powder particles is substantially sphericalor the shape is flaky having particle size being several tens μm, thesilver powder can be used as a core material.

The silver powder used as the core material may be substantiallyspherical, flaky or flat, and if the production conditions inconventionally-adopted production process are modified, particle sizedistribution of sharpness and dispersibility can be ensured, to somedegree. Thus, once silver powder is used in a form of the silver powderincluding silver particles to which fine silver particles adhere, it hasexcellent handleability and does not require any large amount ofprotective colloid when formed into a paste, though silver nanoparticleshave poor dispersibility in themselves. And a silver paste preparedusing such silver powder can contain silver particles by an amount beingequivalent to an amount of conventional silver pastes, resulting inmaking it possible to thickening coating when a wiring pattern of acircuit is drawn.

In the silver powder made of silver particles, each to which fine silverparticles adhere as above-described, its sinterable temperature is 170°C. or less and it shows extraordinarily excellent sinteringcharacteristics. Thus, if a silver paste (silver ink) is produced usingthe the silver powder made of silver particles, each to which finesilver particles adhere, and the wiring patterns of a circuit are drawnusing such a silver paste (silver ink), a coating thickness can beobtained which is sufficient to provide a circuit that can flow a largeamount of current. And what is more, the ease of sintering the powderparticles substantially improves the characteristics of the silver paste(silver ink) as a conductor, such as low electric resistance andcontinuity reliability.

In the silver powder made of silver particles, each to which fine silverparticles adhere according to the present invention, the silver powderis used as a core material. Accordingly, the particle size anddispersibility of the powder particles, each to which fine silverparticles adhere have a much more tendency to depend on the silverpowder as the core material. In other words, it is preferable to select,as the core material, a silver powder which ensures a particle sizedistribution and dispersibility at a high level. Also it is morepreferable to select silver powder containing few impurities, whichwould be a hindrance when lowering electric resistance. So, the presentinventors have made eager research efforts toward the size-refinement ofeach of particles of silver powder itself and finally obtained thesilver powder having the powder characteristics as below. And theinventors have arrived at an idea that excellent products can beobtained by producing the silver powder made of silver particles, eachto which fine silver particles adhere using the above-described silverpowder as the core material.

The above-described silver powder basically has the following powdercharacteristics: a. the average particle size of primary particlesD_(IA) obtained by the analysis of the images of scanning electronmicroscope is 0.6 μm or less; b. the degree of agglomeration representedby D₅₀/D_(IA), where D_(IA) is the above-described average particle sizeof primary particles and D₅₀ is the average particle size obtained by alaser diffraction/scattering particle size distribution measurementmethod, is 1.5 or less; and c. the crystallite size is 10 nm or less.And another type of silver powder has not only the above-describedpowder characteristics a to c, but the following powder characteristic:d. the content of organic purities is 0.25% by weight in terms of amountof carbon. These two types of silver powder are different in content ofpurities due to their different production conditions. The silver powderhaving such powder characteristics has dispersibility at a level whichcannot be obtained by conventional production processes. In thefollowing, the powder characteristics of the silver powder used as thecore material will be described.

The characteristic of a is that the average particle size of primaryparticle D_(IA) obtained by the analysis of the image of a scanningelectron microscope (SEM) is 0.6 μm or less. The term “the averageparticle size of primary particles D_(IA) obtained by the analysis ofthe images of scanning electron microscope” is the average particle sizeof silver powder obtained by analyzing the images observed with ascanning electron microscope (SEM) (preferably the fine silver powderused in the present invention is observed at ×10,000 magnification whileconventional silver powder at ×3,000 to ×5,000 magnification). In theimage analysis of fine silver powder observed with a SEM, round particlesize analysis is conducted by using IP-1000 PC of Asahi Engineering Co.,Ltd. and setting the roundness threshold value and the overlap degree at10 and 20, respectively to obtain the average particle size D_(IA). Theaverage particle size D_(IA) obtained by analyzing the images ofobserved fine silver powder will represent a reliable value of theaverage particle size of primary particles, because it is obtaineddirectly from the observed SEM images. Almost all the D_(IA) values ofthe fine silver powder used in the present invention fall in the rangeof 0.01 μm to 0.6 μm as far as the inventors observe; however inreality, silver powder having much smaller particle sizes may also beconfirmed. That is why the minimum value is not shown clearly.

The characteristic of b is described by the “degree of agglomeration”,which is an index of the dispersibility of particles, because the finesilver powder used as a core material in the present invention showsexcellent dispersibility which conventional silver powder has never had.The “degree of agglomeration” herein used means the value represented byD₅₀/D_(IA), where D_(IA) is the above-described average particle size ofprimary particles and D₅₀ is the average particle size obtained by alaser diffraction scattering particle size distribution measurementmethod. The D₅₀ means that the particle size at volume accumulation of50% obtained by laser diffraction scattering particle size distributionmeasurement method and its value is calculated not by directly observingeach particle size, but by considering an agglomerated powder particleas a single particle (an agglomerate). This is because, in reality, theparticles of silver powder do not lie in the monodisperse state, whereparticles are completely separated from each other, but typically instate where a plurality of particles agglomerate. However, the less,powder particles agglomerate and the more, they lie closely in themonodisperse state, the smaller the average particle size D₅₀ becomes.The D₅₀ of the fine silver powder used in the present invention is inthe range of 0.25 μm to 0.80 μm, which fine silver powder produced by aconventional production process has never had. In the laser diffractionscattering particle size distribution measurement method hereindescribed, the average particle size of fine silver powder is measuredwith a laser diffraction scattering particle size distribution meter,Micro Trac HRA Model 9320-X100 (by Leeds+Northrup) after mixing 0.1 g offine silver powder made of ion-exchanged water and dispersing the silverpowder made of an ultrasonic homogenizer (Nippon Seiki Co., Ltd.) forfive minutes.

On the other hand, “the average particle size of primary particlesD_(IA) obtained by the analysis of the images of scanning electronmicroscope” is the average particle size of silver powder obtained byanalyzing the images observed with a scanning electron microscope (SEM),it represents a reliable value of the average particle size of primaryparticles obtained without considering over the particles in theagglomerated state.

Thus, the inventors take the value calculated by D₅₀/D_(IA), whereD_(IA) is the average particle size obtained by image analysis and D₅₀is the average particle size obtained by laser diffraction scatteringparticle size distribution measurement method, as the degree ofagglomeration. Specifically, according to the above-described theory,the value D₅₀, which reflects the existence of agglomerates, should belarger than the value D_(IA), assuming that the values D₅₀ and D_(IA)can be measured with the same accuracy in the fine silver powder of thesame lot. And the value D₅₀ becomes infinitely close to the value D_(IA)with decreasing agglomerates of the fine silver powder particles; as aresult, the value D₅₀/D_(IA), the degree of agglomeration, becomesinfinitely close to 1. At a stage where the degree of agglomerationbecomes 1, it is said that the silver powder is monodisperse powderwhere no agglomerate of powder particles exists.

Then, the inventors examined the correlation between the degree ofagglomeration of the fine silver powder has and the viscosity of thepaste produced using the fine silver powder as well as the correlationbetween the degree of agglomeration and the smoothness of the conductorobtained by sintering the paste, with fine silver powder havingdifferent degrees of agglomeration. The examination revealed that thereis very excellent correlation between the above factors. This indicatesthat viscosity of a paste produced using fine silver powder can befreely controlled by controlling the degree of agglomeration the finesilver powder has. The examination also revealed that if the degree ofagglomeration is kept 1.5 or less, then the fluctuations in theviscosity of the paste of the fine silver powder and in the smoothnessof the conductor obtained by sintering the silver powder can also bekept in a very narrow range. It can also be said that the less, theagglomerates of fine silver powder, the more, the film density of theconductor obtained by sintering the fine silver powder is improved,thereby making it possible to lower an electrical resistance ofconductor formed by sintering the silver powder.

However, in reality, the calculated value of the agglomeration degreecan sometimes be less than 1. This can be possibly because thecalculation is made under the assumption that the value D_(IA) used forthe calculation of the agglomeration degree is the value of a truesphere. In theory, the degree of agglomeration cannot be 1, but inreality fine silver particles are not true spheres, thereby yielding acalculated value which is less than 1.

The characteristic of c is that the crystallite size is 10 nm or less.The crystallite size and the sinterable temperature mutually have veryclosely relationship. Specifically, comparing different types of silverpowder having the substantially same average particle size, those havingsmaller crystallite size are more capable of being sintered at lowtemperatures. Thus, fine silver powder having large surface energy dueto its fineness and having a crystallite size being 10 nm or less, justlike the fine silver powder according to the present invention, makes itpossible to lower the sinterable temperature of the fine silver powderitself, as the core material. The reason why the lower limit of thecrystallite size is not provided here, is that certain measuring errorscan be produced depending on the measuring device, measuring conditionsor the like. In addition, it is difficult to require extreme reliabilityof measured values in the range where the crystallite size is less than10 nm. The lower limit dares to be set, the inventors would say, basedon their study, that the lower limit would be about 2 nm.

The characteristic of d is that the content of organic impurities in thesilver powder is 0.25% or less by weight in terms of amount of carbon.Here the carbon content is used as an index of the content of organicimpurities and as a measure of the amount of impurities adhering to thefine silver powder particles. The carbon content was measured withEMIA-320V manufactured by Horiba, Ltd. by a combustion-infraredabsorption method in such a manner as to mix 0.5 g of fine silverpowder, 1.5 g of tungsten powder 1.5 g and 0.3 g of tin powder andthereafter put the mixture into a porcelain crucible. The carbon contentin silver powder obtained by a conventional production process is morethan 0.25% by weight even if washing the silver powder is reinforced,too much.

The fine silver powder according to the present invention is directed toa fine silver powder that conventional production process has neverproduced, in terms of having its powder characteristics: a to c; or a tod. In view of characteristics of sintering temperature, the fine silverpowder used as the core material in the present invention is capable ofstarting a sintering process at as low temperatures as 240° C. or less.The term “sinterable temperature” herein used means the lowesttemperature at which a circuit can be drawn on an alumina substrate witha silver paste prepared using silver powder can undergo sintering by aresistor-measurable extend. Although the lower limit of the sinterabletemperature has not been particularly specified, either, considering thestudies by the present inventors and the common knowledge in this art,it is almost impossible for the fine silver powder, as the corematerial, itself to be sintered at temperatures lower than 170° C. Thus,170° C. can be taken as the lower limit of the sinterable temperature.

The effect of the above-described powder characteristics the fine silverpowder according to the present invention has is to increase the tapdensity of the fine silver powder to as high as 4.0 g/cm³. The “tapdensity” herein used means the density determined in such a manner as toweigh 200 g of fine silver powder accurately, put the weighed powderinto a 150-cm³ measuring cylinder, tap the measuring cylinder byrepeating the 40 mm-stroke dropping of the cylinder by 1,000 times, andmeasure the volume of the fine silver powder. Powder has a high tapdensity when it has a theoretically fine particle size and is in thehighly dispersed state where its particles are not agglomerated.Considering the tap density of conventional silver powder is less than4.0 g/cm³, the above-described tap density value has proved that thefine silver powder according to the present invention is very fine andexcels in dispersibility.

The aforementioned fine silver powder, which is used as the corematerial, is a very fine powder and excels in dispersibility, therebyhaving low-temperature sintering performance allowing the sinterabletemperature of the fine silver powder itself to be 240° C. or lower. Andsilver powder made of silver particles made to adhere thereto, which isvery fine and excels in dispersibility, compared with conventionalsilver powder, and has a low-temperature sintering performance thatallows the sinterable temperature to be 170° C. or lower, is produced bymaking silver nanoparticles adhere to the surface of the particles ofthe above fine silver powder. A ratio of a size of each of the silvernanoparticles to a size of the core material is substantially 1/5 to1/100. The ratio thereof is not limited especially. Namely, the ratio ischanged corresponding to various conditions such as pH, temperature,rate of reaction, and the like regarding as used solutions.

<Process for Producing Silver Powder Made of Silver Particles Each towhich Fine Silver Particles Adhere>

In the following the process for producing silver powder made of silverparticles each to which fine silver particles adhere according to theinvention will be described in terms of two major types: productionprocess 1 and production process 2. The fine silver powder suitably usedin the production processes 1 and 2 will be described separately in thesection “Process for producing fine silver powder”.

Production Process 1:

This production process is “a process for producing silver powder madeof silver particles each to which fine silver particles adhere,characterized by bringing silver powder into contact with a solutioncontaining a silver complex which is obtained by mixing silver nitrateand a complexing agent and dissolving the mixture during stirring; andadding a reducing agent to the resultant solution to allow fine silverparticles to be precipitated on the surface of each silver powderparticle”.

When making the silver powder into the above slurry, the amount ofsilver powder contained is not particularly limited. However, unless theamount of silver powder in the slurry is defined, the amount ofchemicals used in the production cannot be clearly specified. Thus, theproduction process 1 will be described as a process for producing silverpowder made of silver particles each to which fine silver particlesadhere, wherein silver nanoparticles are made to adhere to the surfaceof each silver powder particle in a slurry of 50 g of silver powderdispersed in 1 liter of deionized water. The production process is basedon the assumption that the average particle size of primary particlesD_(IA) of the silver powder used, which is obtained by the imageanalysis of the scanning electron microscope, is 1 μm or less.

First, the “solution containing a silver complex which is obtained bymixing silver nitrate and a complexing agent and dissolving the mixturewhile stirring” will be described. To treat the above-described amountof silver powder, 8 g to 26 g of silver nitrate is used. If the amountof silver nitrate used is less than 8 g, a practically sufficient rateof coating with fine silver particles cannot be obtained, whereas evenif the amount of silver nitrate used is more than 26 g, the coating ratecannot be improved. The complexing agent used is a sulfite or anammonium salt. When using potassium sulfite, the amount used is in therange of 50 g to 150 g. If the amount of potassium sulfite added is lessthan 50 mg, the complexation of silver does not fully progress, andtherefore, a complete silver complex cannot be formed. On the otherhand, more than 150 mg of potassium sulfite well exceeds a sufficientamount for forming a silver complex, and adding the excess amount ofpotassium sulfite does not accelerate the complexiation, resulting inbeing not economical. The solution containing a silver complex isobtained by dissolving the above-described amount of silver nitrate in 1liter of deionized water, adding a complexing agent to the solution andfully stirring the mixed solution.

Then, 50 g of silver powder is added to the solution containing a silvercomplex obtained as above and the slurry of silver powder is fullystirred. A reducing agent is then added to the slurry to cause areduction reaction, so that fine silver powder of nano-order particlesize is allowed to be precipitates uniformly on the surface of eachsilver powder particle. The reducing agent used here is hydrazine, DMAB,SBH, formalin or hypophosphoric acid. When using hydrazine, 5 g to 50 gof hydrazine is dissolved in 200 ml or less (including 0 ml) ofdeionized water and the prepared solution is added within 60 minutes(including the case where the solution is added for a very short time).In the present description, it is noted that “an operation that solutionis added for a very short time” is meant by an operation of establishinga chemical reaction between different solutions as rapid as possible. Ifthe amount of hydrazine added is less than 5 g, the reduction does notsufficiently progress, and the surface of each of silver powderparticles of the silver powder cannot be coated uniformly with finesilver powder. The addition of more than 50 g of hydrazine does notespecially accelerate the reduction reaction and is merely uneconomical.

The solution temperature during the reduction reaction lies in the rangeof a room temperature to 45° C. If the solution temperature is higherthan 45° C., the reduction reaction progresses so rapidly that theprecipitation of fine silver powder on the surface of each silver powderparticle is likely to be non-uniform, resulting in inferior particlesize distribution of the resultant silver powder made of silverparticles each to which fine silver particles adhere. Preferably, thetime the addition of a reducing agent takes is about 5 minutes to 40minutes in the above-described reducing agent concentration range. Ifthe reaction time is shorter than 5 minutes, the produced powderparticles tend to be agglomerated more strongly, whereas if for theaddition of a reducing agent, it takes not shorter than 40 minutes, asatisfactorily uniform coating can be obtained.

After allowing fine silver powder to precipitate on the surface of eachsilver powder particle through reduction reaction in the above-describedmanner, the silver powder is separated through a filter, washed,dehydrated, and dried to yield silver powder made of silver particleseach to which fine silver particles adhere according to the presentinvention. The separation through a filter, washing, dehydration, anddrying may be carried out by various procedures, and the procedure andconditions employed are not limited to any specific ones.

Production Process 2:

This production process 2 is “a process for producing silver powder madeof silver particles each to which fine silver particles adhere,characterized by including: adding silver nitrate and a neutralizingagent into a slurry of silver powder in a dispersing medium; dissolvingthe slurry mixture while stirring to allow fine silver oxide particlesto be precipitated on the surface of each silver powder particle;washing the resultant silver powder; and exposing the silver powder madeof silver oxide particles on its surface to UV rays to reduce the finesilver oxide particles to fine silver particles”.

In the above slurry of silver powder, the amount of silver powdercontained is not particularly limited. However, unless the amount ofsilver powder in the slurry is not clearly defined, the amount ofchemicals used in the production cannot be specified. Thus, theproduction process 2 will be described as a process for producing silverpowder made of silver particles each to which fine silver particlesadhere, wherein silver nanoparticles are made to be adhered to thesurface of each of silver powder particles in a slurry of 50 g of silverpowder dispersed in 1,500 g of dispersion medium. The production processis based on the assumption that the average particle size of primaryparticles D_(IA) of the silver powder used, which is obtained by theimage analysis of the scanning electron microscope, is 1 μm or less.

First, the “slurry of silver powder added to a dispersing medium” willbe described. The dispersion medium used here is ethylene glycol(including monoethylene glycol, diethylene glycol and triethyleneglycol), butanediol (including 1,4-butanediol, 1,2-butanediol and2,3-butanediol) or glycerin. Here, the case where ethylene glycol isused as the dispersion medium will be described. Accordingly, the slurryof the silver powder is prepared by adding 50 g of silver powder to1,500 g of ethylene glycol and stirring the mixture.

Silver nitrate and a neutralizing agent are added to the slurry ofsilver powder obtained as above and dissolved while stirring to allowfine silver oxide particles to be precipitated on the surface of eachsilver powder particle. Preferably, the silver nitrate and theneutralizing agent added are in the form of a solution of silver nitrateor neutralizing agent. The reason is that to do so prevents themaldistribution of chemicals in the slurry of silver powder and allows aneutralization reaction to uniformly occur in the slurry of silverpowder. Preferably used is an aqueous solution of silver nitrateprepared by dissolving 2.50 g to 33.34 g of silver nitrate in 500 g ofdeionized water (equivalent to silver nitrate concentration of 3% byweight to 40% by weight). If the amount of silver nitrate added is lessthan 2.50 g, a sufficient amount of silver oxide to coat the surface ofeach particle of the above-described silver powder uniformly will not beprecipitated. Even if the amount of silver nitrate is more than 33.34 g,the amount of silver oxide adhering to the surface of each silver powderparticle does not change very much, and rather resulting in bringingabout inferior particle size distribution and dispersibility of thepowder particles.

The neutralizing agent may be an alkali metal salt such as sodiumhydroxide and potassium hydroxide. It goes without saying that theamount of the neutralizing agent added depends on the amount of silvernitrate to be neutralized. Assuming that sodium hydroxide is used, theamount is selected from those in the range of 0.588 g to 7.840 g tocorresponding amount of silver nitrate. Sodium hydroxide is alsopreferably used in the form of an aqueous solution. Thus, a solution ofsodium hydroxide in 500 g of deionized water is used.

After allowing fine silver oxide particles to be adhered to the surfaceof each silver powder particle in the above-described manner, the finesilver oxide particles are washed. This washing has to be carried out toremove the solution having been used for the neutralizing reaction andalso fully remove water. Accordingly, it is most preferable to employwashing in water and washing in alcohol in combination. In order to washthe silver powder made of fine silver oxide particles adhering theretoobtained under the above-described conditions, at least 500 g or more ofwater or the largest possible amount of water is used. It is preferablyto dehydrate the washed silver powder. This is done to remove impuritiesas much as possible. In order to ensure that water is removed, washingin alcohol is carried out. For the washing in alcohol, ethyl alcohol,methyl alcohol or isopropyl alcohol may be used. The amount of alcoholused is not particularly limited, as long as the amount used issufficient to remove water.

After completion of the washing, the silver powder made of fine silveroxide particles adhering thereto is immediately exposed to UV rayswithout being dried. Silver powder made of silver particles adheringthereto can be obtained by reducing the fine silver oxide particles onthe surface of each silver powder particle to fine silver particles.Exposure to UV rays accelerates the conversion of the fine silver oxideparticles to fine silver particles and prevents non-uniform reductionfrom occurring. For UV rays used, their wavelengths are not strictlylimited and, for example, UV light used for sterilization can be used.After completion of the exposure to UV rays, the silver powder is fullydried to produce silver powder made of silver particles each to whichfine silver particles adhere according to the present invention.

Process for Producing Silver Powder Preferably Used as Core Material:

A process will be described for producing silver powder (of nearlyspherical shape) suitably used as the core material for the silverpowder made of silver particles each to which finer silver particlesadhere according to the present invention. The production processdescribed here is for producing silver powder having the above-describedpowder characteristics: “a. the average particle size of primaryparticles D_(IA) obtained by the image analysis of the scanning electronmicroscope is 0.6 μm or less”; “b. the degree of agglomerationrepresented by D₅₀/D_(IA), where D_(IA) is the above-described averageparticle size of primary particles and D₅₀ is the average particle sizeobtained by laser diffraction scattering particle size distributionmeasurement method, is 1.5 or less”; and “c. the crystallite size is 10nm or less.” Accordingly, the silver powder described here can be saidto a fully-fine powder in comparison with conventional silver powderproduced using an aqueous solution of silver nitrate.

The process for producing the silver powder is a process including:preparing an aqueous solution of a silver complex by mixing and reactingan aqueous solution of silver nitrate and a complexing agent; making anorganic reducing agent contact with the above aqueous solution forreaction to allow silver particles to be precipitated by reduction;filtering the precipitate-containing solution; washing the resultantsilver powder; and drying the same, characterized in that the reducingagent, silver nitrate and the complexing agent are added in such amountsthat allow each of the above chemicals to be more dilute after theaddition. It is conventionally common that a solution of a reducingagent and an aqueous solution of a silver complex are mixed for a veryshort time in a bath and the concentration of silver is generally kept10 g/l or more. Thus, silver nitrate, a reducing agent and a complexingagent have needed to be added in large amounts to ensure productivitywhich balances the scale of facilities. In the following, the productionprocess will be described more specifically, taking an example of theprocess in which aqueous ammonia is used as a complexing agent.

The most important characteristic of the production process according tothe present invention is in that after the contact reaction of anaqueous solution of an ammine complex and an organic reducing agent, theconcentration of the organic reducing agent is low, and therefore, theamount of the organic reducing agent remaining adsorbed on the surfaceof the produced silver powder particle or the amount of the organicreducing agent taken in the inside of each of the powder particlesduring their growing process can be reduced. Accordingly, in thesolution produced by mixing a solution of a reducing agent and anaqueous solution of a silver complex, it is most preferable to keep theconcentration of the organic reducing agent be 1 g/l to 3 g/l, whilekeeping the concentration of silver be 1 g/l to 6 g/l.

The concentration of silver has a proportional relationship withrelative to the amount of the reducing agent, and it goes without sayingthat the higher the concentration of silver becomes, the larger amountof silver powder can be obtained. However, if the concentration ofsilver exceeds 6 g/l, precipitating silver particles tend to be coarser,and the resultant silver powder has a particle size not different fromthat of conventional silver powder. Thus, fine silver powder having anexcellent dispersibility, at which the present invention aims, cannot beobtained. On the other hand, if the concentration of silver is less than1 g/l, very fine silver powder is certainly obtained; however, too finesilver powder absorb a larger amount of oil and causes the viscosity ofits paste to be increased. This in turn requires a larger amount ofvehicle to be added, and in the sintered conductor formed as a finalproduct, its film density is low and its electrical resistance tends tobe increased. And besides, industrial productivity required cannot besatisfied.

To obtain fine silver powder of the present invention in a good yield,an optimum requirement condition is to keep the concentration of theorganic reducing agent be 1 g/l to 3 g/l, while keeping theconcentration of silver be 1 g/l to 6 g/l. The concentration of theorganic reducing agent in the range of 1 g/l to 3 g/l is selected as anoptimum range for obtaining fine silver powder in the relationship withthe silver concentration of an aqueous solution of a silver/amminecomplex. If the concentration of the organic reducing agent exceeds 3g/l, the amount of the reducing solution added to the aqueous solutionof a silver/ammine complex is certainly decreased; however, theagglomeration of the powder particles of the silver powder precipitatedthrough reduction starts to progress significantly and the amount ofimpurities (herein the amount of impurities is taken as the carboncontent) contained in the powder particles starts to be increasedrapidly. If the concentration of the organic reducing agent is less than1 g/l, the total amount of the reducing solution used increases, therebyincreasing the amount of a waste water to be treated. This does not meetthe industrial economy.

The term “organic reducing agent” herein used means hydroquinone,ascorbic acid, glucose or the like. Of these organic reducing agents,hydroquinone is preferably selectively used in the present invention.Hydroquinone excels in reactivity compared with other organic reducingagents in the present invention and can bring about an optimum reactionrate for obtaining silver powder of small crystallite size and lowcrystallizability.

Other additives can also be used in combination with the above-describedorganic reducing agent. The term “additives” herein used means gluessuch as gelatin, amine-group polymers, celluloses or the like. Additivesare desirably selected which stabilize the process of the precipitationof silver powder through reduction, and at the time, perform a certainfunction as a dispersant. Any proper additive can be selectively useddepending on the type of organic reducing agent, process or the like.

In the process for contact reacting the aqueous solution of asilver/ammine complex and the reducing agent obtained as above toprecipitate fine silver powder through reduction, it is desirable to usea process in the present invention in which, as shown in FIG. 2, acertain pass through which an aqueous solution S₁ of a silver/amminecomplex is flowed through (hereinbefore and hereinafter referred to as“first pass”) and a second pass b which joins the first pass a midwayalong the pass are provided, an organic reducing agent and optionallyadditives S₂ are flowed into the first pass a through the second pass bso that S₁, the organic reducing agent and optionally S₂ are contactmixed at the juncture m of the first pass a and the second pass b toprecipitate silver particles through reduction (hereinafter referred toas “joining mixing process”).

Employing such joining mixing process makes it possible to complete themixing of the two solutions for a shortest-mixing time and allow thereaction to progress while keeping the reaction system uniform, whichleads to the formation of uniformly shaped powder particles. Further, ifthe solution after the mixing contains a decreased amount of organicreducing agent, the amount of the organic reducing agent remainingadsorbed on the surface of each particle of the fine silver particleprecipitated through reduction is also decreased. This makes it possibleto decrease the amount of impurities attached on the fine silver powderobtained through flitration and drying. The decrease in the amount ofimpurities attached on the fine silver powder in turn makes it possibleto lower an electric resistance of the sintered conductor formed of asilver paste which uses the silver powder.

Further, when obtaining an aqueous solution of a silver/ammine complexby contact-reacting an aqueous solution of silver nitrate and aqueousammonia, it is desirable to use an aqueous solution of silver nitratewhose silver nitrate concentration is 2.6 g/l to 48 g/l and yield anaqueous solution of a silver/ammine complex whose silver concentrationis 2 g/l to 12 g/l. Specifying the concentration of an aqueous solutionof silver nitrate is, in other words, specifying the amount of theaqueous solution of silver nitrate. Considering over the silverconcentration of an aqueous solution of a silver/ammine complex beingkept 2 g/l to 12 g/l, the concentration and amount of aqueous ammonia tobe added thereto are inevitably determined. Although the technologicalreasons have not been clarified yet at present, use of an aqueoussolution of silver nitrate whose silver nitrate concentration is 2.6 g/lto 48 g/l makes it possible to obtain a fine silver powder having thebest production stability and stable quality.

The resultant fine silver powder is then washed and dried to yieldsilver powder as a core material. The washing may be carried out usingwashing in water and washing in alcohol in combination or using washingin alcohol alone. Washing process is not particularly limited. Thedrying may also be carried out by any suitable process.

To provide the fine silver powder not only with the above-describedcharacteristics a to c, which are obtained by the above-describedproduction process, but also with the characteristic d. the content oforganic impurities is 0.25% by weight or less in terms of amount ofcarbon”, the washing process must be changed. In the following, thewashing process will be described.

To decrease the amount of impurities contained in the fine silverpowder, washing carried out at the final stage is very important. Thewashing may be carried out either by using washing in water and washingin alcohol in combination or by using washing in alcohol alone; however,the washing is reinforced in washing in alcohol. Thus, washing iscarried out for 40 g of fine silver powder by using about 100 ml ofdeionized water and then using about 50 ml of alcohol. However, in thepresent invention, when carrying out washing in alcohol, 200 ml or moreof alcohol is used for 40 g of fine silver powder, in other words, 1 kgof fine silver powder is washed using an excessive amount of alcohol,that is, 5 L or more of alcohol.

An amount of impurities by reinforcement of washing can be decreasedjust because, in the contact reaction between an aqueous solution of asilver/ammine complex and a reducing agent, the present inventionemploys a technique for keeping the amount of organic reducing agentremaining in the solution after mixing small by employing alow-concentration-reaction system of low concentration of organicreducing agent.

The silver powder made of silver particles each to which fine silverparticles adhere according to the present invention has alow-temperature sintering performance at a level which conventionalsilver powder has never had, because it is constructed by making toadhere fine silver particles (silver nanoparticles) to the surface ofeach of the silver powder particles of the silver powder. Further, itcan have a particularly excellent low-temperature sintering performance,because the silver powder is very fine, excellent in dispersibility andhas few impurities, which conventional silver powder has never had, asits core material.

In the meantime, the process for producing silver powder made of silverparticles each to which fine silver particles adhere according to thepresent invention is excellent in running stability through theproduction process, and therefore, it can produce silver powder made ofsilver particles to each which fine silver particles adhere veryeffectively. The process can also produce silver powder used as the corematerial effectively, because it employs a process for producing silverpowder using the above-described dilute solution.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a powder particle of silverpowder made of silver particles each to which fine silver particlesadhere; and

FIG. 2 is a view showing the concept of mixing of an aqueous solution ofa silver/ammine complex and a reducing agent.

BEST MODE FOR CARRYING OUT THE INVENTION

In the following, the preferred embodiment of the present invention willbe described in detail, comparing with Comparative Examples. In thefollowing examples, silver powder used as a core material was producedfirst. Then, silver powder made of silver particles each to which finesilver particles adhere, a silver paste using the above silver powdermade of silver particles each to which fine silver particles adhere, andtest circuits using the silver paste were produced. The specificresistance and sinterable temperature were measured for the producedtest circuits.

EXAMPLE 1

Process for Producing Silver Powder Used as a Core Material:

In this example, first silver powder (whose particle is spherical) usedas a core material was produced. The production process was as follows.

First, 63.3 g of silver nitrate was dissolved in 9.7 liter of deionizedwater to prepare an aqueous solution of silver nitrate. Then, 235 ml of25% by weight aqueous ammonia was added for a very short time to theaqueous solution of silver nitrate and stirred to give an aqueoussolution of a silver ammine complex.

The aqueous solution of a silver ammine complex was introduced into afirst pass a having an inner diameter 13 mm, shown in FIG. 2, at a flowrate of 1500 ml/sec and a reducing agent was allowed to flow through asecond pass b at a flow rate of 1500 ml/sec so that the solution and theagent came into contact with each other at a juncture m while being keptat 20° C. to precipitate fine silver powder through reduction. Thereduction agent used here was a solution of 21 g of hydroquinone in 10liter of deionized water. Accordingly, the concentration of hydroquinoneat the time of completion of the mixture was about 1.04 g/l, which isvery low concentration.

To superlatively pick up 40 g of the resultant fine silver powder, thesolution was filtered using a nutsche, and the separated fine silverpowder was washed with 100 ml of water and 600 ml of methanol and driedat 70° C. for 5 hours to yield fine silver powder.

The powder characteristics of the silver powder obtained as abovecorrespond to those of Comparative Example 1, which are shown in Table 1together with those of the other examples and Comparative Examples. Nowthe term “sinterable temperature” herein used will be described, becauseits contents, such as measuring method, has not been clearly defined bythe description so far. The term “sinterable temperature” shown in Table1 means that the lowest temperature at which a silver paste is producedusing each of the silver powder and each having been used for drawing awiring pattern of a circuit on an alumina substrate can be sintered intoproducts to such an extend that the electrical resistance of thesintered products is measurable. The sintering temperature was selectedfrom the temperatures in the rage of 150 to 250° C. Then, specificresistance was measured for the circuits 1 mm width pattern-circuitwhich was obtained by sintering the silver pastes. To judge whether thesintering was done well or not, the sintered state was also observedwith a scanning electron microscope. The composition of silver pasteswas: 85 wt % of fine silver powder and 15 wt % of terpineol. FIBanalysis was used for measuring the size of the precipitated crystalgrains to determine the crystallite size.

Production of Silver Powder Made of Silver Particles Each to which FineSilver Particles Adhere:

Silver powder made of silver particles each to which fine particlesadhere was produced in accordance with the above-described productionprocess 1 using the silver powder, as a core material obtained as above.

First, “a solution containing a silver complex which is obtained bymixing silver nitrate and a complexing agent and dissolving the mixturewhile stirring” was prepared so as to make fine silver particles adhereonto the surface of each particle of 50 g of the above-described silverpowder. The solution containing a silver complex was prepared by firstdissolving 17 g of silver nitrate in 1 liter of deionized water and thenadding 86 g of potassium sulfite, as a complexing agent, to thesolution.

Then, 50 g of the above-described silver powder was added to thesolution containing a silver complex obtained as above and stirred for 1minute. A reducing agent was added to the above silver powder-containingsolution so that a reduction reaction was caused to precipitate finesilver powder of nano-order particle size uniformly. The reductionreaction was carried out by adding a solution of 10 g of hydrazine in 90ml of deionized water, as a reducing agent, for a very short time at asolution temperature of 40° C. for 10 minutes. After fine silverparticles were precipitated by reduction on the surface of each powderparticle through reduction in the above-described manner, theprecipitate was separated through a filter, washed, dehydrated and driedto yield silver powder made of silver particles each to which finesilver particles adhere according to the present invention.

The powder characteristics of the silver powder made of silver particleseach to which fine silver particles adhere obtained as above weredetermined in the same manner as in the case of the silver powder usedas a core material. Then, a silver paste was prepared using the abovesilver powder and a test circuit was formed using the silver paste.Then, the specific resistance and sinterable temperature were measuredfor the formed test circuit. The resultant characteristics are shown inTable 1 as those of Example 1.

EXAMPLE 2

Process for Producing Silver Powder Used as a Core Material:

In this example, first silver powder (which is substantially spherical)used as a core material was produced. The production conditions were asdescribed below.

Silver powder was produced under production conditions different fromthose of Example 1 and the powder characteristics of the resultantsilver powder were determined. Then, a silver paste was prepared usingthe above silver powder and a test circuit was formed using the silverpaste. Then, the specific resistance of the conductor and sinterabletemperature were measured for the formed test circuit.

First, 63.3 g of silver nitrate was dissolved in 3.1 liter of deionizedwater to prepare an aqueous solution of silver nitrate. Then, 235 ml of25% by weight aqueous ammonia was added for a very short time to theaqueous solution of silver nitrate and stirred to give an aqueoussolution of a silver ammine complex.

The aqueous solution of a silver ammine complex was introduced into afirst pass a having an inner diameter of 13 mm, shown in FIG. 2, at aflow rate of 1500 ml/sec and a reducing agent was allowed to flowthrough a second pass b at a flow rate of 1500 ml/sec so that thesolution and the agent came into contact with each other at a juncture mwhile being kept at 20° C. to precipitate fine silver powder throughreduction. The reduction agent used here was a solution of 21 g ofhydroquinone in 3.4 liter of deionized water. Accordingly, theconcentration of hydroquinone at the time of completion of the mixturewas as low as about 3.0 g/l, which is very low concentration.

To superlatively pick up 40 g of the resultant fine silver powder in thesame manner as in Example 1, the solution was filtered using a nutsche,and the separated silver powder was washed with 100 ml of water and alarge volume of, that is, 600 ml of methanol and dried at 70° C. for 5hours to yield fine silver powder as a core material. The powdercharacteristics of the silver powder obtained as above correspond tothose of Comparative Example 2, which are shown in Table 1 together withthose of the other examples and Comparative Examples.

Production of Silver Powder Made of Silver Particles Each to which FineSilver Particles Adhere:

Silver powder made of silver particles each to which fine silverparticles adhere was produced in accordance with the above-describedproduction process 2 using the silver powder obtained as above as a corematerial. First, a slurry of silver powder was prepared by adding 50 gof the above silver powder to 1500 g of ethylene glycol, as a dispersionmedium, and fully stirring the mixture to disperse the silver powder inthe medium.

Then, silver nitrate and a neutralizing agent were added to theresultant slurry of silver powder and dissolved by stirring toprecipitate fine silver oxide particles on the surface of each silverpowder particle. Silver nitrate was added first using an aqueoussolution of 16.67 g of silver nitrate in 500 g of deionized water(equivalent to 30 wt % silver nitrate concentration). Then aneutralizing agent was added and fully stirred. The neutralizing agentused was a solution of 3.92 g of sodium hydroxide in 500 g of deionizedwater. Fine silver oxide particles were made to adhere to the surface ofeach silver powder particle in this manner.

The silver powder made of fine silver oxide particles adhering theretowas then separated by filtration and washed. The washing was carried outusing washing in water and washing in alcohol in combination. Firstwashing in water was performed. The silver powder made of fine silveroxide particles adhering thereto obtained under the above-describedconditions was washed in 500 g of water to remove impurities on thepowder as much as possible and dehydrated. Then, to ensure that water isremoved, the powder was washed in 500 g of isopropyl alcohol.

Immediately after completion of the washing, the silver powder made offine silver oxide particles adhering thereto was exposed to UV rayswithout being dried to reduce the fine silver oxide particles on thesurface of each silver powder particle to fine silver particles. Theexposure to UV rays was conducted using CL15-A by Toshiba Corporation,which is usually used as a bactericidal lamp, for 3 hours so as toaccelerate the rapid conversion of the fine silver oxide particles tofine silver particles and prevent the occurrence of non-uniformreduction. After that, drying was carried out by conventional procedureto yield silver powder made of silver particles each to which finesilver particles adhere according to the present invention on whichlittle impurities were attached.

The powder characteristics of the silver powder made of silver particleseach to which fine silver particles adhere obtained as above weredetermined in the same manner as in the case of the silver powder usedas a core material. Then, a silver paste was prepared using the abovesilver powder and a test circuit was formed using the silver pastes.Then the specific resistance and sinterable temperature were measuredfor the formed test circuit. The resultant characteristics are shown inTable 2 as those of Example 2.

EXAMPLE 3

Process for Producing Silver Powder Used as a Core Material:

In this example, first silver powder (of nearly spherical shape) havinga large crystallite size was produced using the process shown below, andthe powder characteristics of the resultant silver powder weredetermined. Then, a silver paste was prepared using the above silverpowder and a test circuit was formed using the silver paste. Then thespecific resistance and sinterable temperature were measured for theformed test circuit.

First, 20 g of polyvinyl pyrrolidone was dissolved in 260 ml ofdeionized water and 50 g of silver nitrate was dissolved to prepare anaqueous solution of silver nitrate. Then, 25 g of nitric acid was addedfor a very short time to the above solution and stirred to yield asilver-containing nitric acid solution. At the time of completion of themixing, the concentration of ascorbic acid was about 36.0 g/l.

A reducing solution was prepared by adding and dissolving 35.8 g ofascorbic acid, as a reducing agent, in 500 ml of deionized water.

The silver-containing nitric acid solution was put into a reaction bathand then the above reducing solution was also added for a very shorttime to the reaction bath. Silver powder was precipitated throughreduction by stirring the mixed solution, while keeping the solutiontemperature 25° C., to cause a reaction.

The resultant fine silver powder was separated by filtration using anutsche, and the separated silver powder was washed with 100 ml of waterand 500 ml of methanol and dried at 70° C. for 5 hours to yield silverpowder as a core material. The powder characteristics of the silverpowder obtained are shown in Table 1 as those of Comparative Example 3.

Production of Silver Powder Made of Silver Particles Each to which FineSilver Particles Adhere:

Silver powder made of silver particles each to which fine silverparticles adhere was produced in the same manner as in theabove-described Example 2 using the silver powder obtained above as acore material. To avoid the repetition, the detailed description of theproduction process will be omitted here.

The powder characteristics of the resultant silver powder made of silverparticles each to which fine silver particles adhere were determined inthe same manner as in the case of the silver powder used as a corematerial. A silver paste was produced using the silver powder made ofsilver particles each to which fine silver particles adhere and a testcircuit was formed using the silver paste. The specific resistance andsinterable temperature were measured for the test circuit. The resultsare shown in Table 1 as those of Example 3.

EXAMPLE 4

Process for Producing Flake Silver Powder Used as a Core Material:

In this example, flake-shaped silver powder was produced by machiningsilver powder which is substantially spherical, and the resultant silverpowder was used as a core material. The powder characteristics of theflake silver powder are shown in Table 2 as those of Comparative Example4.

Production of Flake Silver Powder Made of Silver Particles Each to whichFine Silver Particles Adhere:

Flake silver powder made of silver particles each to which fine silverparticles adhere was produced in the same manner as in theabove-described production process 1 using the flake silver powderobtained above as a core material. The production conditions employedwere the same as those of Example 1. To avoid the repetition, thedetailed description of the production conditions will be omitted here.

The powder characteristics of the resultant flake silver powder made ofsilver particles each to which fine silver particles adhere weredetermined in the same manner as in the case of the flake silver powderused as a core material. A silver paste was produced using the silverpowder made of silver particles each to which fine silver particlesadhere and a test circuit was formed using the silver paste. Thespecific resistance and sinterable temperature were measured for thetest circuit. The results are shown in Table 2 as those of Example 4.

COMPARATIVE EXAMPLES Comparative Example 1

The silver powder described in Example 1 and used as a core material wasalso used as Comparative Example. The powder characteristics etc. of thesilver powder are shown in Table 1 as those of Comparative Example 1.

Comparative Example 2

The silver powder described in Example 2 and used as a core material wasalso used as Comparative Example. The powder characteristics etc. of thesilver powder are shown in Table 1 as those of Comparative Example 2.

Comparative Example 3

The silver powder described in Example 3 and used as a core material wasalso used as Comparative Example. The powder characteristics etc. of thesilver powder are shown in Table 1 as those of Comparative Example 3.

Comparative Example 4

The silver powder described in Example 4 and used as a core material wasalso used as Comparative Example. The powder characteristics etc. of thesilver powder are shown in Table 1 as those of Comparative Example 4.

<Comparative Examination of Examples and Comparative Examples>

The above-described Examples 1 to 3 and Comparative Examples 1 to 3 arecompared while referring to Table 1.

[Table 1] TABLE I Powder Charactaristics Silver Powder made of SilverParticles, each to Sinterd Conductor Core Material which Fine SilverCharactaristics Tap Crystallite Particles adhere Spesific Sinterable SSADensity D₅₀ D_(max) D_(IA) Size Carbon D₅₀ D_(max) SSA ResistanceTemparature Sample m²/g g/cm³ μm D₅₀/D_(IA) nm Content % μm m²/g μΩ · cm° C. Example 1 2.54 4.2 0.31 0.97 0.30 1.03 7 0.32 0.29 0.97 3.73 3.4150 Example 2 1.68 4.7 0.55 1.86 0.49 1.12 7 0.21 0.57 1.85 2.85 5.3 150Example 3 0.62 4.0 3.03 11.0 1.20 2.53 38 0.22 3.26 11.0 0.99 7.9 150Comparative 2.54 4.2 0.31 0.97 0.30 1.03 7 0.28 — 7.9 180 Example 1Comparative 1.68 4.7 0.55 1.86 0.49 1.12 7 0.21 — 5.9 190 Example 2Comparative 0.62 4.0 3.03 11.0 1.20 2.53 38 0.30 — not 250 Example 3available

Comparing the powder characteristics of the silver powder as a corematerial and those of silver powder made of silver particles each towhich fine silver particles adhere for the cases of Example 1 andComparative Example 1, Example 2 and Comparative Example 2, and Example3 and Comparative Example 3 while referring to Table 1, it is apparentthat even if fine silver particles are made to adhere to the corematerial, the powder characteristics of the core material are hardlychanged. Particularly from the fact that there was almost no change inthe value D_(max) before and after the adhesion of fine silverparticles, it is apparent the dispersibility which the silver powderused as the core material has is maintained in the silver powder made ofsilver particles each to which fine silver particles adhere. Thisindicates that a silver powder as fine as possible having excellentdispersibility is advantageously used as the core material. The reasonof this is that the crystallite size of the silver powder as the corematerial is kept unchanged, though the crystallite size of the silverpowder made of silver particles each to which fine silver particlesadhere is not described in the Tables.

Comparing the sintered conductor characteristics for the cases ofExample 1 and Comparative Example 1, Example 2 and Comparative Example2, and Example 3 and Comparative Example 3, it is obvious thatsinterable temperature is so decreased by making fine silver particlesadhere to silver powder as the core material that conventional knowledgecannot explain. Particularly in the cases of Examples 1 to 3, though thepowder characteristics of both core material and silver powder made ofsilver particles each to which fine silver particles adhere aredifferent from example to example, the sinterable temperature is 150° C.for all the cases. On the other hand, in the cases of ComparativeExamples 1 to 3, there exists no fine silver particle layer on thesilver powder, the sintered conductor characteristics are largelyaffected by the powder characteristics, and in the case of ComparativeExample 3, it is impossible to measure the specific resistance. Thecomparison so far confirms that the silver powder made of silverparticles each to which fine silver particles adhere according to thepresent invention is not affected by the powder characteristics of thecore material and it can be sintered at low temperatures, because of thefine silver particles adhering to the surface of each of particles ofthe silver powder as a core material.

Then, the above-described example 4 and Comparative Example 4 will becompared while referring to Table 2. For the flake silver powder as corematerial and the flake silver powder made of silver particles each towhich fine silver particles adhere, the powder characteristics are drawnwhich are different from those of the nearly sphere-shaped powderparticles shown in Table 1. For the flake powder, the measurement ofcrystallite size and carbon content was omitted because its crystallitesize undergoes changes by physical machining and its surface iscontaminated by lubricants used in the machining. The value D_(IA)obtained by the observation with a scanning electron microscope is alsoexcluded from measurement items, because of its large fluctuations in afield of view. Instead, the measurements by laser diffraction scatteringparticle size distribution measurement method are often used.

[Table 2] TABLE II Powder Charactaristics Flake Silver Powder made ofSilver Flake Silver Powder Particles, each to which Sinterd Conductor asCore Material Fine Silver Particles Adhere Charactaristics Tap TapSpesific Sinterable SSA Density D₁₀ D₅₀ D₉₀ D_(max) SSA Density D₁₀ D₅₀D₉₀ D_(max) Resistance Temparature Sample m²/g g/cm³ μm m²/g g/cm³ μm μΩ· cm ° C. Example 4 0.29 2.6 8.91 16.7 28.9 67.9 0.72 2.7 9.17 18.8 29.874.0 21 200 Comparative — 109 200 Example 4

Comparing the powder characteristics of flake silver powder as the corematerial of Comparative Example 4 and those of flake silver powder madeof silver particles each to which fine silver particles adhere ofExample 4 with reference to Table 2, it is apparent that even if finesilver particles are made to adhere to the core material, the powdercharacteristics of the core material do not change very much, just likethe case of the powder of the substantially spherical particles shown inTable 1. The value D_(max) seems to be increased after the adhesion offine silver particles; however, it cannot be necessarily asserted thatthere exist very large fluctuations, in view of including any measuringerrors.

Comparing over the sintered conductor characteristics of Example 4 withComparative Example 4, it is apparent that sinterable temperature isdecreased by making fine silver powder adhere to the core material. Thisis the same as in the cases of Examples 1 to 3. The comparison so farconfirms that the flake silver powder made of silver particles each towhich fine silver particles adhere according to the present invention isnot affected by the powder characteristics of the core material and itcan be sintered at low temperatures, because of the fine silverparticles adhering to the particle surface of the flake silver powder asa core material.

INDUSTRIAL APPLICABILITY

The silver powder made of silver particles each to which fine silverparticles adhere according to the present invention is constructed bymaking fine silver powder (silver nanoparticles) adhere to the surfaceof each silver powder particle. Such construction enables the silverpowder of the present invention to exhibit a low-temperature sinteringperformance at a level which conventional silver powder has never had.Because of its stable low-temperature sintering performance, whichconventional silver powder has never had, a drastic expansion ofapplications in which silver powder is used is expected and a drasticreduction in energy cost during the sintering process will be madepossible. Further, use of the silver powder, which is very fine, excelsin dispersibility and contains less impurities compared with anyconventional silver powder, as the core material for the silver powdermade of silver particles each to which fine silver particles adheremakes it possible to realize an especially excellent low-temperaturesintering performance and the formation of a low-resistant sinteredconductor.

In the meantime, the process for producing silver powder made of silverparticles each to which fine silver particles adhere according to thepresent invention is excellent in running stability through theproduction process, and therefore, it can produce silver powder made ofsilver particles each to which fine silver particles adhere veryeffectively. Thus, it can provide inexpensive and high-quality silverpowder in the market, thereby contributing to the expansion ofapplications in which the silver powder made of silver particles each towhich fine silver particles adhere according to the present invention isused.

1. Silver powder made of silver particles, each whose center part isregarded as a core material characterized in that: silver particles eachbeing finer than the silver particle of said center part adheres to saidcore material.
 2. The silver powder made of silver particles each towhich fine silver particles adhere according to claim 1, wherein thesilver powder is substantially spherical.
 3. The silver powder made ofsilver particles each to which fine silver particles adhere according toclaim 2, wherein the silver powder has the following powdercharacteristics a to c: a. an average particle size of primary particlesD_(IA) obtained by image analysis of a scanning electron microscope is0.6 μm or less; b. a degree of agglomeration represented by D₅₀/D_(IA),where D_(IA) is said average particle size of primary particles and D₅₀is the average particle size obtained by laser diffraction scatteringparticle size distribution measurement method, is 1.5 or less; and c. acrystallite size is 10 nm or less.
 4. The silver powder made of silverparticles each to which fine silver particles adhere according to claim2, characterized in that the silver powder has the following powdercharacteristics a to d: a. an average particle size of primary particlesD_(IA) obtained by image analysis of scanning electron microscope is 0.6μm or less; b. a degree of agglomeration represented by D₅₀/D_(IA),where D_(IA) is said average particle size of primary particles and D₅₀is the average particle size obtained by laser diffraction scatteringparticle size distribution measurement method, is 1.5 or less; c. acrystallite size is 10 nm or less; and d. a content of organicimpurities is 0.25% by weight or less in terms of amount of carbon. 5.The silver powder made of silver particles each to which fine silverparticles adhere according to claim 1, wherein each of particles of thesilver powder is substantially flat.
 6. The silver powder made of silverparticles each to which fine silver particles adhere according to claim1, wherein sinterable temperature is 170° C. or less.
 7. A process forproducing the silver powder made of silver particles each to which finesilver particles adhere according to claim 1, comprising the steps of:bringing a silver powder into contact with a solution containing asilver complex, which is obtained by mixing silver nitrate and acomplexing agent and dissolving the mixture while stirring; and adding areducing agent into the solution to allow fine silver particles to beprecipitated on the surface of each silver powder particle.
 8. Theprocess for producing the silver powder made of silver particles each towhich fine silver particles adhere according to claim 7, wherein thecomplexing agent is a sulfite salt or an ammonium salt.
 9. A process forproducing the silver powder made of silver particles each to which finesilver particles adhere according to claim 1, comprising the steps of:adding a silver nitrate and a neutralizing agent into a slurry of silverpowder in a dispersing medium; dissolving the slurry mixture whilestirring to allow fine silver oxide particles to be precipitated on thesurface of each silver powder particle; washing the resultant silverpowder; and irradiating with UV rays to reduce the fine silver oxideparticles to fine silver particles.
 10. The process for producing thesilver powder made of silver particles each to which fine silverparticles adhere according to claim 9, wherein the neutralizing agent isany one or two or more selected from the group consisting of sodiumhydroxide, potassium hydroxide and aqueous ammonia.
 11. A process forproducing the silver powder made of silver particles each to which finesilver particles adhere according to claim 1, comprising the step of:using a silver powder of nearly spherical powder particles which isobtained in the steps of: preparing an aqueous solution of a silvercomplex by mixing and reacting an aqueous solution of silver nitrate anda complexing agent; contact-mixing an organic reducing agent with saidaqueous solution of a silver complex; allowing silver particles to beprecipitated by reduction in the solution after the mixing, whilekeeping the silver concentration at 1 g/l to 6 g/l and the organicreducing agent concentration at 1 g/l to 3 g/l; separating theprecipitated silver particles through a filter; and washing the silverparticles in water and then in an alcoholic solution.
 12. A process forproducing the silver powder made of silver particles each to which finesilver particles adhere according to claim 1, comprising the steps of:using silver powder of nearly spherical powder particles which isobtained in the steps of: preparing an aqueous solution of a silvercomplex by mixing and reacting an aqueous solution of silver nitrate anda complexing agent; contact-mixing an organic reducing agent with saidaqueous solution of a silver complex; allowing silver particles to beprecipitated by reduction in the solution after the mixing, whilekeeping the silver concentration at 1 g/l to 6 g/l and the organicreducing agent concentration at 1 g/l to 3 g/l; separating theprecipitated silver particles through a filter; and washing the silverparticles in water and then in an excess amount of alcoholic solution.